1. Field of the Invention
This invention relates to shaping and cooling glass sheets and particularly to the high speed production of bent glass sheets that are toughened by air quenching, and most particularly, for shaping and heat treating relatively thin glass sheets, particularly those having a nominal thickness of 1/8 inch (3.2 mm) or less. Thinner glass sheets sag more readily than thicker glass sheets at any given elevated temperature above the glass deformation temperature. Hence, it is more difficult to control the shape imparted to thinner glass sheets.
Shaped glass sheets are widely used as side windows in vehicles such as automobiles or the like and, to be suitable for such application, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side windows are installed. It is also important that the side windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area. During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increasing the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of a glass sheet to breakage, tempering also causes a glass sheet to fracture into relatively small, relatively smoothly surfaced fragments that are less injurious than the relatively large, jagged fragments that result from the more frequent breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel-type furnace where the glass sheet is one of a series of sheets that are heated to the deformation temperature of glass and along an extension of said path into a shaping station where each glass sheet in turn is transferred onto a lifting mold that lifts the glass sheet into adjacency to a vacuum mold. The vacuum mold lifts and holds the shaped glass by suction. The lifting mold retracts to below the substantially horizontal path. At about the same time, a transfer and tempering ring having an outline shape conforming to that of the glass sheet slightly inboard of its perimeter moves upstream into a position below the vacuum mold and above the lifting mold. Release of the vacuum deposits the shaped glass sheet onto the tempering ring. The tempering ring conveys the glass sheet into a cooling station for rapid cooling.
In prior art apparatus, the vacuum mold was kept elevated at the same raised position it occupied to provide clearance for entry and removal of the tempering ring into and out of the shaping station. Consequently, it was necessary to lift the lifting member a relatively considerable distance above the level of said substantially horizontal path to bring the heat-softened glass sheet sufficiently close to the upper vacuum mold to enable the vacuum mold to complete the transfer of the glass sheet thereto by suction. The time needed for this lifting step over said relatively considerable distance limited the maximum rate of production of shaped glass sheets to less than a desired rate.
2. The Prior Art
U.S. Pat. No. 1,889,881 to Ralph C. Thompson shapes a soft, horizontally disposed glass sheet between an upper mold and a lower mold of complementary shape. A portion of the lower mold within its outline marginal portion is recessed below the surface of the outline marginal portion to avoid simultaneous engagement of the opposite glass sheet surfaces in the vision area of the glass sheet. The soft glass sheet is simultaneously engaged at its opposite surfaces in the marginal portion only and sags within the limits of the recessed lower mold portion inside the marginal portion.
U.S. Pat. No. 3,508,903 to Samuel L. Seymour press bends tong gripped glass sheets between molds having shaping surfaces of slightly different radii of curvature to avoid simultaneous engagement of the viewing area between the molds. Such an arrangement does not avoid tong marks in the bent glass.
U.S. Pat. No. 3,607,187 to Harold A. McMaster lifts a soft, flat glass sheet by lowering a vacuum mold toward said sheet and applying suction through a downwardly facing curved shaping surface of a vacuum mold to shape the sheet by suction thereagainst. Much power is needed to provide the suction necessary to shape the entire flat glass sheet to conform to the curved shape of the vacuum mold in incremental portions, particularly those portions most widely spaced from the vacuum mold when other portions are initially engaged by the vacuum mold.
U.S. Pat. No. 3,846,104 to Samuel L. Seymour provides method and apparatus in which glass sheets are conveyed through a furnace on conveyor means, and heated while passing through the furnace to a temperature approaching the glass softening point. At a shaping station beyond the furnace, each glass sheet in turn is lifted by a lower outline shaping mold which raises the glass sheet into engagement with an upper vacuum mold having a shape conforming to that desired for the glass. The upper vacuum mold remains at the shaping station and holds the shaped glass thereagainst as the lower shaping mold retracts to below the level of the conveyor means. A tempering ring shaped to support the bent glass sheet adjacent its marginal or peripheral edge only, moves generally horizontally between the shaping station and a cooling station to receive each shaped glass sheet released by the vacuum mold and transfer it to the cooling station. The upper vacuum mold remains in an elevated position after it releases the vacuum until such time as the tempering ring leaves the shaping station for the cooling station and a subsequent glass sheet arrives at the shaping station. Therefore, each glass sheet must be lifted to said elevated position for transfer to said vacuum mold, a time consuming step.
U.S. Pat. No. 4,092,141 to Robert G. Frank and DeWitt W. Lampman provides similar apparatus with vertically movable sheet transfer means for rapidly removing from the tempering ring each bent glass sheet after its surfaces harden sufficiently to permit it to be conveyed on an additional downstream conveyor. The latter provides a glass sheet supporting surface at an elevation slightly higher than the level at which the glass sheet is supported by the transfer and tempering ring. However, the apparatus of this prior art patent also provides that the upper vacuum mold remain in an elevated position awaiting the arrival of a subsequent glass sheet at the shaping station.
The prior art patents fail to provide a shaping operation that causes the upper vacuum mold to be vertically movable into a parking position that provides clearance for the glass sheet entering the shaping station, and yet reduces the vertical distance that the glass sheet must be lifted until it is close enough to the upper vacuum mold to permit the glass sheet transfer to be completed by suction.